1. Field of the Invention
This invention relates to a photoresist composition and, more particularly, relates to a photoresist composition having improved adhesion to a substrate.
2. Description of the Prior Art:
Photoresist compositions are widely used for plate making and photoetching and must have the following characteristics: (a) high resolution, (b) high sensitivity, (c) stability quality upon preservation, (d) a sufficiently hard photoresist layer to resist corrosion and abrasion, and (e) strong adhesion to substrates. The photoresists further must have a wide latitude in developing, while both the photoresist itself and its developing solution should not give rise to environmental pollution problems.
Conventionally a dichromate series photoresist made of a mixture of dichromate and a water-soluble polymer, for example glue, gelatin, albumin (egg white) and polyvinyl alcohol, is widely used for plate making and photoetching. Dichromate, i.e., Cr.sub.2 O.sub.7.sup.2-, is used to harden the polymer. The reason this composition is used is that this photoresist is cheap and approximately meets the above-mentioned criteria (a), (b), (d) and (e). However, this type of photoresist gradually changes in quality by a natural reaction, the so-called "dark reaction", even though it is normally stored in the dark, and the photoresist becomes unusable. Further, the use of chromium in the composition as a cross-linking agent raises problems, upon disposal, of environmental pollution.
To resolve the above-mentioned drawbacks, a proposal which has been advanced is to make a photoresist composition by introducing a photosensitive radical into the polymeric material itself or by mixing an organic photosensitive agent with a polymeric material. One of these attempts has been the formulation of a photoresist comprising a water-soluble polymer mixed with a light-sensitive organic crosslinking agent instead of dichromate. (See Japanese Patent Publication No. 48,946/1976). An example of the formulation is a photoresist composition comprising a water-soluble bisazide compound which has a hydrophilic radical such as 4,4'-bisazidostilbene-2,2'-disulfonic acid or its salt, and a water-soluble polymer such as polyacrylamide, polyvinylpyrrolidone or gelatin. This bisazide photoresist varies in its characteristics according to the characteristics of the water-soluble polymer with which it is combined. This photoresist approximately meets the above-mentioned criteria (a), (b), (c) and (e). However, this type of photoresist generally weakly adheres to the substrate. Weak adhesion causes low resolution.
Generally, a bisazide photoresist more weakly adheres to a substrate than a dichromate photoresist. The reason for this is unclear. However, it is believed that in a dichromate photoresist a chromium ion acts as a connector for joining the polymer component to the substrate. On the contrary, a bisazide photoresist has no such similar function.
In order to obviate the above-stated problems of biasazide photoresists, methods of improving the adhesion of such photoresists have been proposed. Japanese Patent Publication No. 19,982/1976 discloses a photoresist composition comprising a bisazide compound, a suitable water-soluble polymer and at least one silane compound selected from the group of .gamma.-aminopropyltriethoxysilane, N-(.beta.-aminoethyl)-.gamma.-aminopropylmethyldimethoxysilane and N-(.beta.-aminoethyl)-.gamma.-aminopropyltrimethoxysilane as an adhesive activator. However, the silane compounds do not impart adequate increased adhesion of the bisazide photoresist to a substrate, because these silane compounds have only one alkoxysilane group. A need therefore continues to exist for a method of improving the substrate binding characteristics of bisazide photoresist formulations.